This invention relates to X-ray beam generation devices and more particularly to an improved electron beam target for such devices.
Ordinarily, an X-ray beam generating device, referred to as an X-ray tube, comprises dual electrodes of an electric circuit in an evacuated chamber or tube. One of the electrodes is a cathode electrode and thermionic emitter which is mounted in the tube in spaced apart relationship to a target or anode electrode. The cathode is electrically heated to generate a stream or beam of electrons directed towards the target anode. The electron beam is appropriately focussed as a thin beam of very high velocity electrons striking the target. The target utilizes a striking surface of predetermined material (usually a refractory metal) and a particular geometric shape so that the kinetic energy of the striking electrons against the target material is converted to electromagnetic waves of very high frequency, i.e., X-rays, which emanate from the target to be collimated and focussed for penetration into an object for examination purposes.
Well known primary refractory metals for the target surface area exposed to the impinging electron beam include tungsten and molybdenum and their alloys for improved X-ray generation. In addition, the high velocity beam of electrons impinging the target generates extremely high and localized temperatures together with very high internal stresses leading to deterioration of the target, particularly targets of a composite structure. As a consequence, it has become a practice to utilize a rotating target assembly generally comprising a shaft supported disc-like structure having a concentric annular band of the target metal thereon which is impinged by the electron beam. Ordinarily this annular band is positioned concentrically with the target disc structure and adjacent to its rim periphery with a radial or band thickness significantly less than the inner diameter of the described annular band. The annular band is referred to as the focal track of the impinging electron beam. By means of a rotating target, the impinged region of the target is continuously changing to avoid localized heat concentration and to better distribute the heating effects throughout the target structure. Heating of the target by the impinging electrons is a major problem in X-ray target structures and tends to severely increase stress cracking and general target deterioration. In a high rotational speed target, heating must be kept within certain proscribed limits to reduce thermal stresses in composite target structures as well as to protect low friction high precision bearings which support the target. In the enclosed and evacuated environment of an X-ray tube, a target structure must have a high heat storage capacity since most heat transfer from the rotating target takes place through radiation from the target to the tube or envelope structure. Ordinarily only about 1.0% of the energy of the impinging electrons is converted into X-rays with the remainder converted into heat which must be dissipated from the target. The practice of rotating targets has progressed to target rotational speeds in excess of 10,000 rpm. At such elevated speeds it becomes quite important that a rotating target composite structure with its refractory metal surfaces have intrinsic resistance to high centrifugal forces and elevated temperature in excess of 1200.degree.C. which exacerbate smaller defects for progressive breakdown and target failure.